1. Field of the Invention
The present invention relates to a digital transmission apparatus for subscribers, more particularly to a digital transmission apparatus for subscribers which is disposed between a central office and a plurality of remote subscribers and, at the same time, is mainly constituted by a subscriber side transmission line wired on the remote subscriber side, a plurality of feeder transmission lines wired on the central office side, some of which are unused (protection transmission line) and the remainder of which are currently used (active), and a switch function unit which performs the switching of subscriber time slots with respect to transmission signals communicated between these subscriber side transmission line and feeder transmission lines.
In North America etc., subscribers served by a telephone company will frequently be scattered at remote locations. It is extremely uneconomical to establish a central office for each of these remote subscribers. Therefore, much use is now being made, in a group of remote subscribers, of a transmission mode in which a central office terminal (COT) which multiplexes subscriber analog signals transferred from an exchange, transforms the same to primary order group digital signals, and transmits the same via a common transmission line (feeder transmission line) to the vicinity of the group of remote subscribers is disposed in the central office and in which a remote terminal (RT) disposed in the vicinity of the group of remote subscribers demultiplexes the primary order group digital signals directed to the remote subscribers and distributes the signals to the subscribers via the subscriber side transmission line. The above-described system is a downstream system. For an upstream system from the remote subscribers toward the above-described central office terminal (COT), similarly, subscriber analog signals from the remote subscribers are converted to the digital signal and multiplexed by the above-described remote terminal, reach the above-described central office terminal via the above-described feeder transmission lines, are demultiplexed there, and then are input to the above-described exchange. The central office terminal (COT), feeder transmission lines, remote terminal (RT), and subscriber side transmission line referred to here constitute the abovementioned subscriber system digital transmission apparatus, which is called a "digital loop carrier (DLC)" in North America.
As the above-described feeder transmission lines, copper wires have been mainly used. The wire length usually reaches as long as several tens of kilometers, and therefore there is a high possibility of occurrence of certain transmission faults. Moreover, there also exists a high possibility of occurrence of a transmission fault since the copper wires themselves no longer function as a transmission media due to aged deterioration.
For this reason, in a digital transmission apparatus for subscribers (DLC), it is indispensable to provide an active/protection switch function at least for the feeder transmission lines, so that when a transmission fault occurs in any of the plurality of active feeder transmission lines, that line is smoothly switched to the protection transmission line, to thus maintain normal communication.
2. Description of the Related Art
A detailed explanation will be made later of a conventional digital transmission apparatus for subscribers (DLC) with reference to drawings.
In the current market, demand is now shifting from the conventional digital transmission apparatus for subscribers (DLC) to the next generation digital transmission apparatus for subscribers (NGDLC). There are various characteristics to this NGDLC, the greatest of which are: (1) the structure of the feeder transmission lines is changed from the conventional five- (four active+one protection) to a 28-line structure; (2) the remote terminal (RT) is further provided with a switch function unit for performing the switching of the subscriber time slots; (3) any one or several of the feeder transmission lines among the 28 feeder transmission lines can be freely determined as the protection transmission line. Note that, a detailed explanation will be given later also for the above-described next generation digital transmission apparatus for subscribers (NGDLC) with reference to the drawings.
To handle such a next generation digital transmission apparatus for subscribers (NGDLC), if the active/protection switching mechanism in the conventional digital transmission apparatus for subscribers (DLC) is used as is, the construction of the apparatus is conspicuously enlarged, and therefore there is a problem in that the demand that the remote terminal (RT) should be made as compact as possible cannot be satisfied. This is because, when the active/protection switching mechanism in the conventional digital transmission apparatus for subscribers DLC is used as is, it is necessary to ensure the space for containing enough units for switch-controlling the active/protection transmission lines and units for switching the active/protection transmission lines corresponding to the maximum number of protection transmission lines that can be expected (generally the internal portion of the remote terminal is constituted by a shelf accommodating these units). Also, the reason is that an enormous number of wires for containing subscriber lines multiplexed in 24 channels must be drawn from all of the 28 multiplexer/demultiplexer units on the backboard of the shelf with the assumption that any of the primary order group framing synchronization units and line interface units belonging to the 28 feeder transmission lines may be used by itself as a protection transmission line.